Project 3: Modifiers of Steps in HDPathogenesis Huntington's disease (HD) is a tragic neurodegenerative disorder that is caused by an inherited CAG expansion that extends a variable glutamine tract in huntingtin, leading to the inexorable death of striatal neurons, with incapacitation and early death. The continuing long-term goal of this project is to discover steps in the disease pathway that, when modified, delay the age at onset of HD. We have delineated early disease events in the striatal cells of precise genetic HD replicas, HdhQ111 mice, and now we have discovered that the onset of one of these, nuclear-huntingtin, is dramatically shifted by genetic background. This finding, together with the linkage peaks from our Modifiers of Age at Onset in Pairs of HD Siblings (MAPS) study, points strongly to early HD CAG modifier genes. Thus, our hypothesis is that genes and compounds that modify disease steps in HdhQ111mice may influence HD onset in man. Three specific aims will test this hypothesis. Aim1 will evaluate huntingtin-partners as modifiers. Genetic analyses in HD cohorts will determine if partner loci are associated with unexpectedly early or late onset, highlighting candidate modifier genes and pathways. Partner proteins will be decreased in STHdhQ111 striatal cells, using siRNA, to reveal those that modify disease-measures. Based on our observation that the HD CAG repeat is a major determinant of cellular energy, Aim 2 will evaluate mitochondria as a modifier. The B6.A-mito CSS line will be used to substitute A/J for B6 mitochondria, to determine the effect on the onset of nuclear-huntingtin in HdhQ111 striatum. Libraries will be screened to discover compounds that modulate the low ATP levels in STHdhQ111 cells. Aim 3 will discover genetic modifiers in an unbiased way. Matings with B6.A CSS lines and B6.HdhQ111 mice will substitute individual A/J for B6 autosomes, starting with five chromosomes syntenic to the MAPS linkage peaks. This strategy will reveal chromosomal regions that contain loci that shift the onset of nuclear-huntingtin in striatum and allow us to pursue genetic experiments to reveal their identies. Thus, this project is focused on finding modifiers of early disease in mice that are precise genetic HD replicas, with the goal of providing validated entry points for the development of rational treatments to slow the onset of this devastating disease in individuals who carry the HD mutation.